On Fri, 21 Sep 2012, Leo Song wrote: > If connect an UMS device (for example, an Android phone) to Linux PC and > Windows PC, then copy a big file (~500MB) to the device, you will find that > Linux PC's write speed is much faster than Windows PC. This issue can be fixed > by hiding the "caching mode page" on UMS device. > > Previously in f_mass_storage.c: do_mode_sense(), caching mode page was the > only mode page, and it was configured as: > Write cache enabled (WCE=1) > Read cache not disabled > No cache retention priorities > > The "Write cache enable"(WCE) bit has different influence for Linux PC and > Windows PC. > > For Linux PC, whether WCE is 0 or 1, the write speed from Linux PC to UMS > device has no difference. Linux PC just ignores this flag. That isn't true at all. The flag doesn't affect write speed but it does affect other things. > For Windows PC, if > 1. the UMS devices has a valid "caching mode page" > 2. the WCE bit in "caching mode page" is set to 1 > 3. the "write-cache buffer flushing" on Windows PC is not turned off > > With these three conditions together, Windows PC will send SCSI WRITE COMMAND > with FUA=1 (FUA, force unit access), and FUA=1 will cause UMS device to wait > until the blocks have been written onto the medium, which will drag down write > speed a lot. Unluckily, this is Windows PC's default behavior. > > On Windows, the FUA=1 flag can be changed by turning off "write-cache buffer > flushing" under UMS device's properties, but if we do so, Windows will pop up > error message as following: > > Windows could not change the write-caching setting for the device. > Your device might not support this feature or changing the setting. > [ OK ] > > Windows need to change UMS device's caching mode page, but this page is hard > coded and can't be changed, so there will be error pop up. Right; the mass-storage gadget doesn't support the MODE SELECT command. Even if it did, we really don't want the gadget to bypass the page cache. > To fix this issue, we can just hide the "caching mode page". With this patch, > there will be no influence for Linux PC, because Linux PC does not handle UMS > device's caching mode page; for Windows PC, if it does see the "caching mode > page", it will not send SCSI WRITE COMMAND with FUA=1, and write speed will > become as fast as Linux PC. Your assumptions about Linux are wrong. I don't think hiding this page is a good idea. Can't you get the result you want by loading the driver with the nofua parameter, as Michal suggested? Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
